Spin-phonon coupling and exchange interaction in Gd substituted YFe0.5Cr0.5O3

Journal of Magnetism and Magnetic Materials 447 (2018) 26–31

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Spin-phonon coupling and exchange interaction in Gd substituted YFe0.5Cr0.5O3 Karan Singh, Mohit K. Sharma, K. Mukherjee ⇑ School of Basic Sciences, Indian Institute of Technology Mandi, Mandi, 175005 Himachal Pradesh, India

a r t i c l e

i n f o

Article history: Received 12 July 2017 Received in revised form 18 August 2017 Accepted 17 September 2017 Available online 19 September 2017 Keywords: Magnetic oxides Rare-earth ions and impurities Raman spectra

a b s t r a c t We report the evolution of physical properties due to partial substitution of Gd on the Y site in a mixed metal oxide YFe0.5Cr0.5O3. This compound exhibits negative magnetization at low applied fields. Our investigations on Y1-xGdxFe0.5Cr0.5O3 (x = 0.0, 0.2, 0.4 and 0.6) compounds is carried out through magnetization and Raman spectroscopy studies. It is observed that even with 20% Gd substitution, the negative magnetization observed in YFe0.5Cr0.5O3 is suppressed. Due to magnetic rare earth ion Gd3+, additional exchange interaction of the form Gd-O-Fe/Cr dominates the magnetic interaction arising due to the transition metal ions. This results in positive magnetization in Gd-substituted compounds. Temperature dependent Raman spectroscopy along with magnetization studies revealed that the observed shifts of Raman mode is due to spin-phonon coupling. Hardening of Raman mode observed below 240 K in YFe0.5Cr0.5O3 weakens and softening of phonon modes was observed for Y0.4Gd0.6Fe0.5Cr0.5O3 compound. This implies that additional magnetic interactions due to Gd ions play a dominating role in dictating the behavior of the Gd-substituted compounds. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction In past couple of decades, extensive investigations are being carried out on rare-earth orthochromites and orthoferrites of the form LnBO3 (Ln = rare-earth ions, B = Fe or Cr). These compounds are interesting as they form a platform to study interesting physics associated with the coupling between two magnetic sub-lattices, thereby, providing an insight to study the complex interplay between 3d and 4f electrons [1–7]. Interestingly, in recent years a new family of mixed metal oxides (by combining orthoferrites and orthochromites) of the form RFe0.5Cr0.5O3, was discovered in perovskites [8–12]. Such compounds are important as they exhibit interesting magnetic properties, which can be useful from the viewpoint of technological application and fundamental physics. Combining the two transition metals within the perovskite structure can be an effective approach to enhance the magnetic properties and at the same time tune/induce functional properties as compared to their parent compounds. A single compound, DyFe0.5Cr0.5O3, exhibits both magnetoelectric (ME) coupling as well as magnetocaloric effect (MCE) [10], whereas DyCrO3 shows only MCE [5] while DyFeO3 shows magnetic field induced multiferroicity [1]. It is also reported that it is possible to tune the ME

⇑ Corresponding author. E-mail address: [email protected] (K. Mukherjee). https://doi.org/10.1016/j.jmmm.2017.09.042 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.

coupling strength in DyFe0.5Cr0.5O3 by rare-earth substitution [12] and hole doping [13]. The compound YFe0.5Cr0.5O3 is reported to show the phenomenon of negative magnetization while such features were absent in the end members YFeO3 and YCrO3 [8]. In YFe0.5Cr0.5O3, the rare-earth ion is non-magnetic and the observed negative magnetization arises from the competition between the single ion magnetic anisotropy and antisymmetric Dzyaloshinsky-Moriya interactions [8]. By partial substitution of magnetic rare-earth in of place Y3+, YFe0.5Cr0.5O3 can be a good candidate to study the evolution of magnetic properties due to 4f electrons from Gd3+ ions. Hence with this aim, in this manuscript, we focused on isovalent doping of Y3+ site by Gd3+ ions. Even though there are reports about the magnetic studies of the YFe0.5Cr0.5O3 but to best of our knowledge, investigation of the evolution of magnetic properties of YFe0.5Cr0.5O3 due to Gd substitution is lacking in literature. Here, we report the results of our investigations on Y1-xGdxFe0.5Cr0.5O3 (x = 0.0, 0.2, 0.4 and 0.6) compounds through magnetic and Raman spectroscopic studies. It was observed that the negative magnetization in YFe0.5Cr0.5O3 below 240 K is suppressed even with 20% Gd substitution. It implies that with the magnetic rare earth ion the magnetic coupling Gd-O-Cr/Fe dominates the magnetic interaction arising due to the transition metal ions, resulting in positive magnetization in Gd-substituted compounds. Temperature dependent Raman and magnetization studies gave evidences of

K. Singh et al. / Journal of Magnetism and Magnetic Materials 447 (2018) 26–31

spin-phonon coupling in these compounds. Hardening of Raman modes observed for YFe0.5Cr0.5O3 below 240 K give rise to softening of mode as the concentration of Gd is increased. 2. Experimental details Polycrystalline samples of Y0.8Gd0.2Fe0.5Cr0.5O3 (Gd-0.2), Y0.6Gd0.4Fe0.5Cr0.5O3 (Gd-0.4) and Y0.4Gd0.6Fe0.5Cr0.5O3 (Gd-0.6) are prepared by solid state reaction method under the similar conditions as reported in Ref. [11]. The YFe0.5Cr0.5O3 (Gd-0.0) sample is the same as used Ref [11]. The structural analysis is carried out by X-ray diffraction (XRD, Cu Ka) using Rigaku Smart Lab instruments. The Rietveld refinement of the powder diffraction data is performed using FullProf Suite software. Temperature (T) (2– 390 K) and magnetic field (H) (up to 50 kOe) dependent magnetization are performed using Magnetic Property Measurement System (MPMS) from Quantum design, USA. Raman spectra of the compounds are collected at different temperature (180–300 K) in backscattering geometry by using LabRam HR Evolution with grating 1800, 532 nm wavelength laser is used as an excitation source under low laser power (7 mW) to avoid local heating. 3. Results and discussion Fig. 1 exhibits the XRD pattern for the family of compounds. All the compounds are found to be single phase. The Rietveld refinement confirmed the orthorhombic perovskite structure with Pnma space group. The obtained crystallographic parameters are shown in Table 1. With respect to parent compound, lattice parameters and unit cell volume slightly increases due to the difference in

(a)

Exp. Cal. Exp.-Cal. Bragg position

Gd-0.2

Intensity (a. u.)

(b) Gd-0.4

Gd-0.6 Gd-0.4 Gd-0.2 Gd-0.0

Intensity (a. u.)

(c) Gd-0.6

32.8

20

40

60

33.2 2θ (Degree)

33.6

80

2θ (Degree) Fig. 1. Room temperature Reitveld refined X-ray diffraction patterns for (a) Gd-0.2, (b) Gd-0.4, and (c) Gd-0.6 compounds. Inset of figure (c) shows the expanded XRD pattern of all the compounds. The black curve is for Gd-0.0 compound from Ref. [11].

27

ionic radii of Y and Gd. A comparison plot of XRD pattern of pure and substituted compounds (inset of Fig. 1(c)) demonstrates a shift of diffraction lines to the lower angle indicating that the dopant is being incorporated into the lattice. Fig. 2 shows the temperature response of dc magnetization (v = M/H) for all the compounds in the T range 2–390 K, measured at 100 Oe under zero field cooled (ZFC) and field cooled (FC) condition. It is noted that for Gd-0.0 compound a negative magnetization is observed around 235 K in the FC curve. This arises due to the fact that the net magnetization from coupling between FeAOAFe, CrAOACr and FeAOACr at 100 Oe is aligned opposite to the direction of applied field [8]. The compound is in a canted magnetic state and generally spin canting is strongly dependent on temperature. This canted spin state arises mainly due to the competition between Dzyaloshinskii-Moriya (DM) and the singleion magnetic anisotropy (SIMA) interactions [14]. In Gd-0.0 compound, the rare-earth ion is non-magnetic and the property of the compound is defined only by the transition metal ions and the compound is in canted magnetic state [11]. As Gd is introduced, there is a competition between canted magnetism of transition metal sub-lattice and Gd moments. This implies that presence of 4f electrons at the Y-site tunes the properties of Gd0.0 compound due to interaction between 4f electron from the rare-earth and 3d electrons from the transition metals. Therefore, for Gd-0.2 due to the presence of Gd spins the net magnetization of the compound is aligned in the direction of applied field, resulting in positive magnetization below 235 K (Fig. 2(a)). Hence it can be said that for Gd-substituted compounds, additional exchange interaction of the form Gd-O-Fe/Cr restrict net coupling between FeAOAFe, CrAOACr and FeAOACr, resulting in absence of negative magnetization at 100 Oe. Additionally, the nature of ZFC and FC curves for Gd-0.2 to 0.6 compounds is similar and it is not accordance to that observed for Gd-0.0 compound. The compounds Gd-0.2 to 0.6 display a paramagnetic-like behavior at low temperature. Similar behavior was observed in other Gd based oxides [15,16]. However in the temperature range of 220–260 K (inset of Fig. 1) broad features are observed in the FC magnetization curves for Gd-0.2 and Gd-0.4 compounds. Such features can be attributed to antiferromagnetic type correlation among spins and with lowering of temperature, the paramagnetic contribution from Gd ions plays the leading role in comparison to the antiferromagnetic type correlations among spins. Also with increasing Gd concentration, the nature of anomaly changes as observed for Gd-0.6 compound implying that the paramagnetic contribution increases with Gd concentration. It is also to be noted here that from these magnetization curves, the ordering temperature cannot be determined, due to dominating role of Gd 4f moment. To get some further insight about the magnetic behavior, field response of magnetization is measured up to 50 kOe at low (50 K) and high temperature (250 K) for Gd-0.2, Gd-0.4 and Gd0.6 compounds (shown in Fig. 3(a)-(c)). The weak hysteresis observed at room temperature for Gd-0.2 and Gd-0.4 compound can attributed to the antiferromagnetic type correlations which are dominant in this temperature range. At low temperature as the paramagnetic contribution from Gd ion become dominant, the nature of the curve is that observed for a paramagnet. For Gd-0.6 compound, the 4f moments play a defining role as compared to the transition metal moments and we get a paramagnetic-like behavior both at low and high temperature [16]. Thus the observations from the M (H) curves are in analogy to that observed from the temperature response of magnetization. In order to get a better insight about the magnetic behavior temperature dependent (in the T range of 180–300 K) Raman spectroscopic studies were performed. Fig. 4 shows the Raman spectrum for all the compounds at different temperature in the spectral range of 100–800 cm
1. In perovskites with ideal cubic

28

K. Singh et al. / Journal of Magnetism and Magnetic Materials 447 (2018) 26–31

Table 1 Parameters for all the compounds obtained from the Rietveld refinement of X-ray diffraction data. The parameter for Gd-0.0 compound is added from Ref. [11] for comparison. Parameters

Gd-0.0

Gd-0.2

Gd-0.4

Gd-0.6

a (Å) b (Å) c (Å) V (Å3) Bragg R-factor RF-Factor

5.557 (3) 7.569 (7) 5.263 (2) 221.38 (9) 2.93 2.54 2.11

5.558 (3) 7.582 (4) 5.276 (3) 222.34 (4) 5.69 4.39 1.890

5.561 (3) 7.599 (5) 5.292 (3) 223.65 (7) 6.82 3.76 1.987

5.556 (2) 7.604 (3) 5.297 (2) 223.76 (7) 3.58 3.69 1.791

v2

0.05

0.08

(a)

ZFC Gd-0.0 FC Gd-0.0

0.01

ZFC Gd-0.2 FC Gd-0.2

200

50 K

-0.08

300

ZFC Gd-0.2 FC Gd-0.2

0.0 0.02 ZFC Gd-0.4 FC Gd-0.4

0.8

0.04

3

0.02

2

0.01 ZFC Gd-0.6 FC Gd-0.6

0.4

200

ZFC Gd-0.6 FC Gd-0.6

200

300

-0.2

(b)

0.4

Gd-0.4

0.00

0.0

-0.15 0.3

-0.4

300

0.0 100

0.15

0.8

(c) Gd-0.6

1

ZFC Gd-0.4 FC Gd-0.4

0

M/H (emu/mol)

M/H (emu/mol)

-0.05

(b)

0.0 250 K

M (μB/f.u)

0.00

0.2

0.00

0.5

M (μΒ/f.u)

0.00

(a) Gd-0.2

0.0

0.0

0 400

T (K) Fig. 2. Temperature (T) dependent dc susceptibility (v = M/H) obtained under zerofield-cooled (ZFC) and field cooled (FC) condition at 100 Oe for (a) Gd-0.0 and Gd0.2 compounds (b) Gd-0.4 and Gd-0.6 compounds. Inset of 2 (a): T response of the ZFC and FC curve of the Gd-0.2 compound in the range 190–320 K. Inset of 2 (b) Same figure for Gd-0.4 and Gd-0.6 compounds.

structure, the Raman active mode is forbidden due to symmetry. In our case, all the compounds crystallize in an orthorhombically distorted perovskite structure with space group Pnma. This space group is obtained by an antiphase tilt of the adjacent transition metals octahedral with Glazer’s notation (a
b+a
) with respect to ideal cubic Pm3m perovskite structure [17]. According to the group theory, 24 (7Ag + 5B1g + 7B2g + 5B3g) Raman active modes were predicted in orthorhombic Pnma structure [18]. Out of these 24 Raman active modes, the 1 strong and 5 weak Raman modes were observed. The observed modes were assigned: B3g(1) (146 cm
1), Ag(2) (180 cm
1), B1g(1) (215 cm
1), Ag(3) (274 cm
1), B1g(2) (333 cm
1) and B2g(4) (677 cm
1), with help of Raman mode reported in literature for the compounds DyFe0.5Cr0.5O3, YCrO3, and GdFeO3 [10,19,20]. As the temperature is reduced no significant changes are observed in the spectra of Gd-0.0 compound (Fig. 4(b), (c) and inset of Fig. 4(a)). This is in accordance with the absence of any structural phase transition in this temperature range for Gd-0.0 compound. Similar behavior was also observed for Gd substituted compounds. Additionally, from Fig. 5(a) it can be said that no noteworthy changes (except for shifting in frequency of some of the modes) were observed in the spectra of the Gdsubstituted compounds in comparison to the Gd-0.0 compound. We now investigate the effect of Gd substitution on the observed modes. Fig. 5 shows the Raman shift with Gd substitution for different modes at 300 K. In the low wave number region (100–

-0.8

-0.3 -50

0

50

H (kOe) Fig. 3. (a), (b) and (c) Isothermal magnetization for Gd-0.2, Gd-0.4 and Gd-0.6 compounds respectively at 50 K and 250 K for field cycling of ±50 kOe.

250 cm
1) the observed mode Ag(2) and B1g(1) depends on A (Y/ Gd) – cation vibrations. Due to the mass difference between Y and Gd; a shift in Raman mode is observed. With increasing the mass, frequency associated with A-cation vibration is slightly decreased. Such types of trend follow within the approximation of harmonic oscillator, x = (k/l)1/2 (k = force constant; l = reduced mass) [17]. The frequency of vibration of A-cation is inversely proportional to the square root of reduced mass. The mass increases about 46% from Gd-0.0 to 0.6, resulting in about 25% decrease in frequency. Hence, it can be said that these observed modes are dominated by vibration of the A-cation. In the intermediate wave number region (250–350 cm
1), Ag(3) and B1 g(2) modes show a low-wave number shift with increasing ionic radii of rare earths. However, as the ionic radii of Y3+ (0.89 Å) and Gd3+ (0.938 Å) are close leading to similar orthorhombic distortion, these modes have a shift toward low-wave number. In the higher wave number region (350–700 cm
1), only B2g(4) is active due to the antisymmetric or symmetric stretching of the CrO6 or FeO6 octahedra in half doped orthochromites [7]. It is observed that this mode show a very weak frequency dependence. Raman shift and line shape parameters were calculated for B2g(4) mode and the temperature dependence of these parameters for all the compounds is shown in Fig. 6(a)–(d). For Gd-0.0, a positive shift in frequency of the mode is observed below 240 K (which is near the temperature below which negative magnetization was observed) due to antisymmetric stretching, implying hardening of

K. Singh et al. / Journal of Magnetism and Magnetic Materials 447 (2018) 26–31

B2g(4)

Intensity

(a)

Intensity

Ag(2)

B2g(4)

300 K 260 K

x=0

240 K 180 K 700 800

Gd-0.0

600 B1g(1) Ag(3) B1g(2) Raman shift (cm-1)

B3g(1)

Gd-0.2 Gd-0.4 Gd-0.6 200

400

600

-1

800

Raman shift (cm ) (b)

(c)

Gd-0.0

Gd-0.0 300 K

300 K

phonon frequencies. Similarly, the temperature dependence of phonon line-widths also reveals an anomaly around 240 K. This positive shift in frequency reduces for Gd-0.2 and a negative shift in frequency is observed around 215 K for Gd-0.4 due to softening of phonon modes (shown by arrow in the Fig. 6). This softening of phonon frequencies becoming stronger is observed just below 240 K for Gd-0.6 compound. Similar behavior is also observed in B1g (1) and B1g (2) modes in the low and intermediate wave number region (not shown), implying that the observed Raman modes in the respective compounds have similar temperature dependence. In these types of perovskite compounds, it has been reported that if there is a change of magnetic interactions in compounds phonon frequencies may be affected [21]. The oxygen ions meditate the magnetic exchange interactions among the transition metal and rare earth ions. If an ion is dislocated from its equilibrium position by x, then the crystal potential is given by [22,23]

260 K

220 K

220 K

180 K 250

Intensity

Intensity

2

260 K

180 K

300

-1

350 100

Raman shift (cm )

150

200

-1

250

B1g(1)

680 B2g(4)

660

210 640

-1

Raman Shift (cm )

0.0

0.2

0.4

0.6

Ag(2)

180

330

U ¼ kx =2 þ

B1g(2)

300

150

X J im Si Sm

ð1Þ

im

where k is force constant and the second term arises from spin-spin interactions. Due to this later term an additional perturbation is created in the potential due to which phonon frequency is affected. A general formula for angular shift of frequency due to change in magnetic interaction is given by [24]

Dx  ð1=2mxÞ

Raman shift (cm )

Fig. 4. (a) Raman spectra at 300 K for all the compounds. (b) and (c) and Inset of (a): Temperature dependent Raman spectra of Gd-0.0 compound in the spectral range of 250–350 cm
1, 100–250 cm
1 and 600–800 cm
1 respectively at selected temperatures between 180 and 300 K.

29

X 2 2 d J im =dx hSi :Sm i

ð2Þ

m

where m is the oxygen mass, x is the unrenormalized mode frequency, hSi.Smi and Jim spin–spin correlation function and exchange integral for magnetic ions respectively. Under molecular field approximation, hSi.Smi  M2; where M is the magnetization in unit of Bohr Magnetron of the respective compounds. Fig. 7 represents the frequency shift of this mode relative to its value at 240 K. This shift is scaled with to the square of magnetization (obtained from temperature response of magnetization under FC condition) of the respective compounds below 240 K, except Gd-0.4 where softening of phonon modes starts around 215 K. For the other compounds i.e. Gd-0.0, Gd-0.2 and Gd-0.6, the two parameters below 240 K, scales with each other implying observed hardening and softening of the Raman mode is due to spin-phonon interactions [24,25]. In Gd-0.0 compound, below 240 K the observed positive shift in phonon frequency is due to the dominating AFM interaction among the spins. With Gd substitution, the phonon mediated through rare earth ion and transition metals plays the leading role and changes the behaviour of phonon frequency. For Gd-0.2 compound the hardening mode weakens and for Gd-0.4 compound a softening of modes is observed. In Gd-0.6 compound, a negative shift in phonon frequency below 240 K is observed. This observed change in the behavior of phonon modes can be attributed to the fact that Gd-O-Fe/Cr exchange interaction plays a leading role in determining the behavior of the Gd-substituted compounds. Thus our observation from Raman Studies is in analogy to that observed from magnetization measurements. 4. Summary

270

Ag(3)

0.0

0.0

0.2

0.2

0.4

0.6

Gd-x

0.4

0.6

Fig. 5. Room temperature Raman shift as a function of Gd concentration for different observed modes in the low wavenumber (100–250 cm
1) region. Lower and upper insets: Similar plots for the intermediate (250–350 cm
1) and higher wavenumber (630–680 cm
1) regions.

We have presented a detailed study of the effect partial substitution of Gd on the Y site in a mixed metal oxide YFe0.5Cr0.5O3 through magnetization and Raman spectroscopy studies. It is observed that even with 20% Gd substitution, the negative magnetization observed in YFe0.5Cr0.5O3 below 240 K is suppressed. The resulting positive magnetization arises due to the fact that the magnetic coupling Gd-O-Cr/Fe dominates the magnetic interaction arising due to the transition metal ions. Temperature dependent Raman and magnetization studies gave evidences of spin-phonon

K. Singh et al. / Journal of Magnetism and Magnetic Materials 447 (2018) 26–31

(a)

(b) 672

672

21

(c) 675 20

18

-1

-1

15

Raman Shift (cm )

666

(d) 17

676

-1

20

Line Width (cm )

-1

Raman Shift(cm )

676

Line Width (cm )

30

16

670 16

674 15

200

240 T (K)

280

200

240 T (K)

280

Fig. 6. Temperature (T) dependence of the Raman shift (Left axis and shown by closed circles) and FWHM (Right axis and shown by open triangles) of B2g(4) mode for (a) Gd0.0 compound, (b) Gd-0.2 compound, (c) Gd-0.4 compound and (d) Gd-0.6 compound.

0.7

-7

2.0x10

Gd-0.0

(a)

tional magnetic interactions due to Gd ions plays a dominating role in determining the behavior of the Gd-substituted compounds. Acknowledgements

0.0

The authors acknowledge Juhi Pandey for her help during Raman measurements and IIT Mandi for financial support. The experimental facilities of Advanced Material Research Centre (AMRC), IIT Mandi are also being acknowledged.

0.0

2 Gd-0.2

(M (μB))

(b)

References

2

(ω(Τ) − ω(240))/2π

0.0

-8

4.0x10 0 0.0

Gd-0.6

(c) 0.0

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-0.4 180

210

240

270

300

T (K) Fig. 7. Temperature (T) dependence of the shift of the frequency of the B2g(4) mode with respect to its value at 240 K and T dependence of magnetization for (a) Gd-0.0 compound, (b) Gd-0.2 compound and (c) Gd-0.6 compound.

coupling in these compounds. Hardening of Raman modes observed for YFe0.5Cr0.5O3 below 240 K give rise to softening of mode as the concentration of Gd is increased, implying that, addi-

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